LONDON – Britain's Prime Minister David Cameron says NATO must rethink its long-term relationship with Russia following its "illegal" actions in Ukraine.

Writing to alliance leaders, Cameron says it is clear that Russia views NATO as an adversary. He says NATO must strengthen its ability to respond quickly to any threat, and agree on how to sustain a robust presence in Eastern Europe.

The letter, which was released Saturday, comes six weeks before the meeting in Wales, the first NATO summit in the U.K. since Margaret Thatcher hosted the alliance 1990 as the Cold War was coming to a close.

Cameron says that today's world is more unpredictable as "Russia has ripped up the rulebook with its illegal annexation of Crimea and aggressive destabilization of Ukraine."